1. Field of the Invention
This invention relates to an optical disc produced using a master disc having signals recorded thereon on light exposure, and a manufacturing method thereof.
2. Related Art
An optical disc, as a recording medium for recording signals to high density, is fabricated by stamping of the crests and recesses of a master disc corresponding to signals by, for example, injection molding. For raising the recording density of the so-produced optical disc, it is necessary to shorten the wavelength of the recording light used for forming the crests and recesses corresponding to the recording signals by light exposure on the master disc.
For forming pits by light exposure on a master disc of a digital video disc (DVD) having a recording density not less than five times that of a compact disc (CD), the Ar or Kr laser having a wavelength of 351 nm is used. In an optical system for converging the laser for light exposure, an objective lens having a numerical aperture (NA) of the order of 0.90 is used.
Although the optical system of the light exposure device, constructed with the laser and the objective lens, operates satisfactorily for forming the DVD master disc by light exposure, the laser of a shorter wavelength and an objective lens of a higher value of NA are required for forming the master disc of the optical disc having a recording density higher than the DVD by light exposure.
However, the laser of a shorter wavelength than 351 nm has as yet not reached the practically usable stage. On the other hand, the value of NA of the objective lens currently used is also approaching the limit value that can be obtained in air, such that it is difficult to obtain the objective lens having the NA not less than 0.90.
In addition, for light exposure of the master disc for the optical disc using the laser of a shorter wavelength than 351 nm, it is necessary to use a photoresist that is less stable than the currently used photoresist i.e, it requires stricter temperature and humidity control. Thus, it is feared that a master disc fabricating process which is in need of strict temperature and humidity control significantly different from that used heretofore needs to be introduced to render it extremely difficult to maintain operational stability.
The currently used master disc fabricating process experiences a problem that the width of a pit formed on light exposure depends on the pit length, such that the shorter the pit length, the narrower becomes the pit width. In a DVD maser disc produced on light exposure using an optical system having the laser for light exposure with wavelength of 351 nm and an objective lens having the numerical aperture of 0.90, the pit width is decreased to a negligible extent, even with the shortest pit 0.4 .mu.m in length, because the shortest pit length is not excessively short as compared to the spot diameter of the laser for light exposure converged on the master disc.
However, if desired to produce a master disc for the optical disc higher in recording density than the DVD, the width of the pit having a length as short as 0.4 .mu.m or less is significantly narrower than a pre-set pit width, such that the pit width reduction is no longer negligible.
If the pit formed on an optical disc is too narrow in width, sufficient amplitude (modulation degree) of RF signals reproduced from the shortest pit cannot be obtained. In addition, the center of the amplitude is shifted significantly from the threshold level for the bi-level signal, thus increasing jitter to lower the signal quality. That is, the larger the jitter, the error rate in counting the data (mark) length from the RF signal with clocks is increased.
However, it is difficult to produce a master disc for an optical disc of high recording density exceeding the recording density of the DVD using a conventional light exposure device used for forming the master disc for the optical disc.